The Session Initiation Protocol (SIP) is a signaling protocol that provides a mechanism for a computing device to locate another device it wants to communicate with over a computer network and to establish a communication session therewith. In particular, SIP is an Internet Engineering Task Force (IETF) standard protocol for initiating interactive user-sessions in a number of scenarios. For example, SIP is used for Internet conferencing, telephony, gaming, virtual reality, event notification, and instant messaging. The SIP protocol enables call setup initiation, routing, authentication and other feature messages to endpoints within an internet protocol (IP) domain.
Like HTTP or SMTP, SIP works in the Application Layer of the Open Systems Interconnection (OSI) communications model. As such, SIP can establish multimedia sessions or Internet telephony calls, and modify or terminate them. The SIP protocol can also invite participants to unicast or multicast sessions that do not necessarily involve the initiator. Because the SIP supports name mapping and redirection services, users initiate and receive communications and services from any location and networks are capable of identifying users within the network.
A soft phone application is a SIP user agent (UA) which may be hosted by a home server; the soft phone application may provide an interface to telephony services. The application may act as an ECMA (European Computer Manufacturers Association) computing application which uses services from an ECMA switching system (PBX).